Printing techniques using a press, such as lithographic printing, allow high printing speeds, associated with excellent image quality.
In lithographic printing, accordingly, an image made up of a text or texts or a design or designs on paper or on any other usable printing medium is obtained with the aid of a printing press, which makes it possible to apply to the medium the inked image, reproducing motif previously carried on at least one printing plate. The printing plate is made outside the press from a flexible material, such as a sheet of aluminum, employing a photoengraving process, for instance. This operation consists on the one hand of making zones on the plate outside the press corresponding to an image that is to be obtained on the paper these zones having an affinity for the liquid colorant vehicle (printing ink) that will be used for printing, and on the other hand, zones that have no affinity whatever with the colorant vehicle.
A printing press generally includes the following:
one or more plate cylinders, around each of which a printing plate is mounted and fixed after it has been engraved;
in proximity with each plate cylinder, a blanket, constituted by a cylinder encompassed by a material having an affinity for the ink used, and arranged to be capable of being put in contact with the motif of the plate during the printing operation, enabling the rotational motion of the plate to be transmitted to the blanket, causing a transfer of the ink from the image zones of the plate to the blanket and;
means for putting the printing medium into contact with each blanket and driving this medium at the same speed as the peripheral speed of the blanket.
The printing phase per se consists of inking the plate, with the ink remaining solely on the zones with which it has an affinity, and setting the assembly into motion.
The ink is then transferred from the image zones of the plate to the blanket, and then from the blanket to the printing medium.
The plate thus constitutes an intermediate transfer element. Known techniques use oleaginous inks: The image zones of the plate are oleophilic and hence hydrophobic; the non-image zones of the plate are hydrophilic and hence oleophobic; finally, the material encompassing the blanket is hydrophobic. In the case where an oleaginous ink is used, water or a wetting solution is also placed on the plate and spreads over the hydrophilic zones, preventing the ink from spreading there. As a consequence of the hydrophobia of the blanket, only the ink is transferred from the plate to the blanket and hence to the printing medium.
This technique also enables polychrome printing on the same medium, by using for example inks that lend themselves to subtractive synthesis of the colors (yellow, cyan, magenta).
Nevertheless, this technique is expensive, time-consuming, and troublesome; accordingly, it can be applied only to very long printing runs.
Making a plate is relatively time-consuming and requires a set of complex tools that must be employed prior to the printing phase.
Moreover, printing a monochrome, multi-page piece of work requires the preparation of as many plates as there are pages in the work. Finally, producing a polychrome piece of work is even more complex, because for each polychrome page a plurality of plates must be prepared in order to obtain all the shades desired on this page, with each plate including different motifs each corresponding to one different basic shade of the image. Hence using inks that lend themselves to subtractive synthesis of colors (yellow, cyan, magenta) requires that at least three plates be prepared: one for yellow, one for cyan, one for magenta, and even a fourth for black, which is very difficult to obtain by subtractive synthesis.
The technique using four plates is known as quadricolor printing. The motifs on each plate are determined for instance by a colorimetric analysis of the original, when the process involves a reproduction.
Once each plate is ready, each one has to be mounted and fixed with extreme precision on a different plate cylinder, so that the successive passage of the medium past each one, after inking, makes it possible to obtain a perfect image without misregistration of the colors. The finer the details of the definitive image, the more complicated and time-consuming it is to achieve proper registration.
Once the definitive printing run of a page has been completed, the corresponding plate or plates must be unmounted, and then the process has to start again for the other pages. Otherwise, a printing system must be available that has as many presses as necessary, which considerably increases the costs and does not obviate the necessity of mounting and fixing all the necessary plates beforehand.
In U.S. Pat. No. 5,129,321, assigned to Rockwell International Corporation, a lithographic printing system is described and claimed that makes it possible to use one less plate and hence dispense with its positioning and fixing, but nevertheless this system has the disadvantage of not being perfectly adapted to long printing runs.
The invention described in the aforementioned patent, for printing with the aid of a conventional (oleaginous) ink, consists of replacing the assembly constituted by the engraved plate and the plate cylinder by a simple cylinder on whose periphery a layer forming a substrate of powdered oleophobic material is deposited each time a new motif is to be printed; an image corresponding to the motifs to be printed is formed on this layer with the aid of a hardenable oleophilic material. The intermediate transfer element is accordingly constituted by the substrate of oleophobic material and the zones of oleophilic material carried by this layer.
The oleophobic material is preferably magnetic, and the cylinder is magnetizable, such that the layer of this material is held on the periphery of the cylinder, magnetizing the cylinder.
Depositing the oleophilic material, in a configuration corresponding to the motifs to be printed, is done with the aid of an electronic, electromechanical or electromagnetic transfer device. Data representing the motifs to be carried on the layer of oleophobic material are contained in an electronic memory and are utilized so that the transfer device deposits the oleophilic material solely at the required locations on the layer of oleophobic material. In one embodiment described in the aforementioned patent, the oleophilic material used is a magnetic meltable material; its deposition onto the substrate of oleophobic material is done by magnetodeposition on the cylinder, in a manner like the deposition of magnetic toner in magnetographic printers. To that end, magnetic heads are located in proximity with the cylinder, which makes it possible to create zones on the substrate that have a magnetization enabling them to attract the particles of oleophilic material.
After its deposition, the oleophilic material is melted, which makes it possible to harden it, so as on the one hand to prevent the motifs from becoming deformed and on the other to lend it a certain cohesion with the substrate of oleophobic material; the particles of oleophilic material attach to the particles of oleophobic material. To that end, the system described in the aforementioned pattern further includes, in proximity with the periphery of the cylinder, a melting device for fixing the oleophilic material.
The printing is done as on a conventional press: The cylinder, after having been coated with the layer forming the substrate and the motifs, is set into rotation, then wetted and inked, in such a way that the ink spreads over the motifs and the wetting product spreads over the oleophobic zones, and then the ink is transferred to the printing medium (paper or other) by way of a blanket.
Once the desired printing run of an image has been completed, the cylinder is demagnetized, causing the layer forming the substrate to detach spontaneously from the cylinder, carrying with it the hardened motifs of oleophilic material that it carries. If printing of a different image is desired, then a new substrate is made, on which new motifs are carried and hardened. It is accordingly very fast and less expensive than with conventional presses to make up the motifs corresponding to an image and remove them again.
This apparatus lends itself to polychrome printing, to the extent that positioning of the motifs is done automatically, by an electronic device.
However, it requires the deposition of two types of materials: that constituting a substrate and that constituting the motifs. Depositing the substrate has no other function than that of allowing the easy later removal of the motifs by forced detachment from this substrate.
Moreover, it is not entirely well-suited to long printing runs, because the substrate has a tendency to detach spontaneously, at least in certain regions, during the rotation of the carrier cylinder during the printing phases, since it is held merely magnetically. This means that the copies that leave the press must be checked, and the image (substrate and motifs) must be sometimes reconstituted on the periphery of the cylinder during the printing run.